LCDs (liquid crystal displays) are commonly used in portable devices which are battery operated. By reducing the battery consumption of such displays, the portable devices may be used for longer periods of time before a recharge or battery change is required.
It is often desirable to have only a portion of an LCD active to minimize power consumption. This occurs when a system is in a state where it is known that only some LCD lines need to be refreshed. One common example is a wireless phone in standby mode where only some icons near the top of the screen need to be turned on. In most existing systems, the whole screen is powered all the time, resulting in mediocre power efficiency. Alternatively, two separate controllers can be used on the same LCD, one dedicated to icons and one dedicated to the main display area. This solution results in additional cost, weight and space.
Several additional methods have been recently developed for decreasing the power required for LCD operation. In PCT application WO 96/41253 by S. Ho, published Dec. 19, 1996 and entitled "Power Down Mode for Computer System", power consumption is reduced by shutting down the LCD controller. However, the LCD module itself is always running at full power and at full multiplexing. This particular implementation is only applicable to a special kind of driver integrated circuit which has built in RAM. These drivers are not standard nor widely used because of their expensive price.
European patent application 0 651 367 by K. Kaeko published May 3, 1995 and entitled "Arrangement for Reducing Power Consumption in a Matrix Display Based on Image Change Detection" discloses an LCD in which power consumption is reduced by reducing the contrast level (voltage) of the image. This is an obvious way to reduce display power consumption and probably the least efficient way since the display legibility is reduced as well.
European patent application 0 725 380 by E. Matsuzaki et al. published Aug. 7, 1996 and entitled "Display Control Method for Display Apparatus Having Maintainability of Display-status Function and Display Control System" discloses a system applicable to display devices which are bistable, i.e. displays having a memory effect. This kind of display does not need to be refreshed every frame. Ferro-electric LCDs are one type of displays incorporating this effect. With this method, only portions of the display are rewritten, namely those which have changed. The remaining portions are still on and retain their previous display values. This is not applicable to passive displays which have no memory.